1. Field of the Invention
The present invention relates to a secondary battery and, more particularly, to an electrode assembly of a secondary battery, which is designed to enhance battery safety, and a method for making the same.
2. Description of Prior Art
A secondary battery comprises an electrode assembly disposed within a can containing an electrolyte. The electrode assembly comprises positive and negative electrodes on which positive and negative active materials are deposited respectively. Therefore, the charging and discharging of the second battery is realized by the physical and chemical reactions occurring between active materials and the electrolyte.
In such a secondary battery, a capacity ratio of the negative active material to the positive active material (N/P ratio) is usually maintained within a range of 1.2-1.4 to obtain a reserve for sufficient absorption of ions released from the positive electrode into the negative electrode.
If the N/P ratio is less than 1, a metal oxide material is precipitated within the secondary battery or the electrolyte may leak since ions discharged from the positive electrode cannot be completely absorbed into the negative electrode. This result in the deterioration of the charging and discharging performance or in the explosion of the battery by increased internal pressure.
Secondary batteries are classified into several types depending on their shape: a cylindrical type, a package type and a prism type. For example, the prism type secondary battery, since the electrode assembly is rolled having an oval section, it is very difficult to provide the suitable N/P ratio throughout the entire surface of the electrode assembly.
This will be described more in detail with reference to FIGS. 1-5.
FIG. 1 shows a conventional prism type secondary battery.
The conventional prism type secondary battery comprises a can 4, a roll electrode assembly 2 disposed within the can 4, and a cap assembly 6 close-tightly mounted on an upper end of the can 4.
Rolling a group of plates makes the roll electrode assembly 2. That is, a separator plate, a negative electrode plate, another separator plate, and a positive electrode plate are, in this order, stacked to provide a group of stacked plates. The plate group is then rolled into the roll electrode assembly as shown in FIG. 2.
The cross section of the roll electrode assembly 2 is track-shaped so that it can be inserted into the prismatic rectangular can 4. As shown in FIG. 2, the roll electrode assembly 2 comprises positive and negative electrodes 2a and 2b, and separators 2c disposed between the positive and negative electrodes 2a and 2b. The track-shaped roll electrode assembly 2 has a straight section St and a curved section C.
The N/P ratio is determined depending on which electrode is disposed on the outer surface of the separator 2c. That is, when the curved section of the roll electrode assembly 2 is unrolled in a flat state, as shown in FIG. 3, there is a length difference .DELTA.L between the positive and negative electrodes 2a and 2b.
Therefore, a difference between the amount of the positive active material and the amount of the negative active material can be obtained according to the following equation: EQU A=.DELTA.LXT
where,
A is the difference between the amount of the positive active material and the amount of the negative active material; PA1 .DELTA.L is the length difference between the positive and negative electrodes; and PA1 T is a thickness of the active material of the electrode.
The difference between the amount of the negative and positive active material causes variations in the capacity ratio difference between the negative and positive electrodes adjacent to each other with the separator disposed there between. That is, a region C where the N/P ratio is less than 1 appears on the roll electrode assembly 2 as shown in FIG. 4.
In the region C, the capacity of the positive electrode 2a is larger than that of the negative electrode 2b, as shown in FIG. 5. Therefore, the ions released from the positive electrode 2a are not be completely absorbed into the negative electrode 2b but precipitated as a metal oxide material 32 on a surface of the negative electrode 2b.
However, in the conventional battery, since the thickness of the separator 2c is uniform, the precipitated metal oxide material 10 continues to grow, then may penetrate the separator 2c, and contact the positive electrode 2a as charging and discharging are repeated. This may cause the battery to explode, deteriorating the safety of the secondary battery.